Immunological mechanisms in the pathogenesis of rheumatoid arthritis may be revealed by study of the remarkably dense lymphocytic infiltration in rheumatoid synovial membranes. A method was developed for extraction of viable intralesional lymphocytes by digestion of synovium with collagenase and deoxyribonucleases and B and T cells quantitated in these populations using surface markers of these cells. Adherence of C sensitized erythrocytes to C3 bearing B cells in cryostat tissue sections were also examined. The infiltrate was found to consist predominantly of T lymphocytes. Attempts will be made to assess the functional potential of these lymphocytes. Cytotoxic potential of the intralesional lymphocytes as well as their sensitization to tissue antigens will be investigated in tissue culture.